Deliberate overriding of an existing file to tests

Deliberate overriding via `save_as` metadata should be allowed, even after the
overwrite detection feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.

Added a page and an article, both to override a tag, with very old dates so
it limits the amount of diff in the generated pages.

Overriding feature introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72
Overwrite detection introduced by ff7410ce2ada85b486a67ae11874d60d135ff939
Delib. overriding and overwrite detect. Fixes #938

Make deliberate overriding (*) works with overwrites detection.

(*) first introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72

The following are decided to be deliberate override:
 - articles using the `save_as` metadata
 - pages using the `save_as` metadata
 - template pages (always)

Pelican now exits in the following 2 cases:
 - at least 2 not deliberate writes to the same file name (behaviour introduced
   by the overwrite detection feature ff7410ce2ada85b486a67ae11874d60d135ff939)
 - at least 2 deliberate writes to the same file name (new behaviour)

Also added info logging when deliberate overrides are performed.

Switched to StandardError instead of IOError, thanks to @ametaireau and
@russkel.
Merge pull request #943 from Rogdham/override_save_as_url

Deliberate overriding and overwrite detection. Fixes #938